Field of the Invention and Related Art Statements
The present invention relates to an apparatus for optically reading information out of an optical record medium such as opto-magnetic disk.
In U.S. Pat. No. 4,730,297, there is described a known apparatus for optically reading information out of an opto-magnetic disk. FIGS. 1A and 1B are schematic side and plan views, respectively showing this known apparatus for reading information recorded on an opto-magnetic disk. In this apparatus in order to move a light spot onto a desired information track promptly, the apparatus comprises a stationary unit S including semiconductor laser 1, collimator lens 2, beam splitter 3, analyzer 7, converging lens 8 and photodetector 9, and a movable unit M including light guide member 4 and objective lens 6. The movable unit M is movably secured to the stationary unit S by means of a bearing B such that the movable unit is rotatable about an optical axis of the stationary unit S. This known apparatus is sometimes called a separation and rotation type. A laser light beam emitted from the semiconductor laser 1 is converted into a parallel light beam by means of the collimator lens 2. The parallel light beam is then transmitted through the beam splitter 3 and is made incident upon an opto-magnetic disk 6 via the light guide member 4 and objective lens 5 arranged in the movable unit M. Light reflected by the opto-magnetic disk 6 is made incident upon the beam splitter 3 by means of the objective lens 5 and light guide member 4. The light beam is then reflected by the beam splitter 4 and is made incident upon the photodetector 9 by means of the analyzer 7 and converging lens 8. The photodetector 9 generates a reproduced signal which represents the information recorded in the opto-magnetic disk 6.
The light guide member 4 is formed by a prism having reflection surfaces. Therefore, there is introduced the retardation into the light beam reflected by the reflection surfaces. That is to say, there is produced a phase difference between the P and S polarized light components. Such a phase difference might deteriorate C/N of the reproduced signal. In order to remove the retardation, the light guide member 4 is constructed to have four reflection surfaces 4a to 4d as illustrated in a plan view of FIG. 1B. The reflection surface 4c is arranged to be rotated about an optical axis L1 by 90 degrees with respect to the reflection surface 4a, and the reflection surface 4d is provided to be rotated about an optical axis L2 by 90 degrees with respect to the reflection surface 4b.
In the known optical reading apparatus, the light guide member 4 is formed to include the four reflection surfaces 4a to 4d which are arranged as stated above, it is possible to cancel the phase difference between the P and S polarized light components. However, the construction of the light guide member 4 is liable to be complicated, large in size and expensive in cost. Moreover, the phase difference generated by the beam splitter could not be compensated for at all.
In Japanese Patent Laid-open Kokai Sho No. 62-223842, there is disclosed another known apparatus for optically reading the information recorded in the opto-magnetic disk. In this known apparatus, the light guide member 4 shown in FIG. 1B is formed by a parallelogram prism. In order remove the retardation the two reflection surfaces of the prism are formed by multi film constructions which have such optical characteristics that the retardations produced by the reflection surfaces are canceled out. However, in the known apparatus the thin film constructions are formed such that the retardations are canceled out at a predetermined wavelength. In practice, the phase difference has the wavelength dependency and when the wavelength of the light beam is varied, the phase difference could not be canceled out and C/N of the reproduced signal becomes worse. Therefore, when the wavelength of the laser light emitted by the semiconductor laser 1 fluctuates due to the variation in the output power and operating temperature, C/N of the reproduced signal is deteriorated. Further also in this known apparatus, the phase difference introduced by the beam splitter 3 is not taken into account. It should be noted that C/N of the reproduced signal is represented by the following equation: ##EQU1## wherein Rs denotes a reflectance of the optical system for the information component of the light beam, .delta. a phase difference of the optical system, .theta.k a Kerr rotation angle at the opto-magnetic disk 6 and .delta.e represents an initial phase difference of the optical system including the opto-magnetic disk 6. Therefore, when the phase difference .delta. is increased, C/N of the reproduced signal is decreased.